Printed circuit boards have conventionally been produced by photolithography methods. Photolithography is a method in which a photosensitive resin composition is coated onto a substrate and subjected to pattern exposure for polymerization and curing of the exposed sections of the photosensitive resin composition, and the unexposed sections are removed with a developing solution to form a resist pattern on the board, and after forming a conductor pattern by etching or plating treatment, the resist pattern is released and removed from the board to form a conductor pattern on the board.
In such photolithography methods, the layer composed of the photosensitive resin composition (hereunder referred to as “photosensitive resin layer”) is laminated on the board by a method of coating the board with a photoresist solution and drying it, or a method of laminating the board with a photosensitive resin laminate (hereunder also referred to as “dry film resist”) obtained by successively laminating a support, a photosensitive resin layer and if necessary a protective layer. A dry film resist is most commonly used for production of printed circuit boards.
A method for producing a printed circuit board using a dry film resist will now be explained in brief. First, when a protective layer such as a polyethylene film is present, it is released from the photosensitive resin layer. Next, a laminator is used to laminate the photosensitive resin layer and support on a copper clad laminate or other board, so that the order: substrate, photosensitive resin layer, support is obtained. The photosensitive resin layer is then exposed with ultraviolet rays such as i-line radiation (365 nm) from an ultrahigh pressure mercury lamp through a photomask with a wiring pattern, for polymerization curing of the exposed sections. The support composed of polyethylene terephthalate or the like is then released. Next, the unexposed sections of the photosensitive resin layer are removed by dissolution or dispersion with a developing solution such as a weakly alkaline aqueous solution, to form a resist pattern on the board. The formed resist pattern is then used as a protective mask for a publicly known process such as etching or pattern plating. Finally, the resist pattern is released from the board to produce a board with a conductor pattern, i.e., a printed circuit board.
When a dry film resist is used, the unpolymerized composition disperses in the developing solution during the developing step in which the unexposed sections are dissolved with the weak alkaline aqueous solution. It is known that when this dispersed substance collects it becomes aggregates in the developing solution, and re-adheres to the board causing shorting defects. When the developing solution is circulated through a filter to prevent such aggregates, a large number of aggregates lead to management problems such as increased filter replacement frequency and a shorter interval for washing of the developing machine. It has been attempted to increase the hydrophilicity of the aggregates by adding a hydrophilic group such as an ethylene oxide chain to the monomer or polymer, or to use a nonylphenol-type monomer to reduce aggregates (Patent document 1), but the problems have not been completely solved by these measures. A novel photosensitive resin composition that can prevent generation of aggregates in developing solutions has therefore been long desired.
Moreover, increased resolution is being demanded for photosensitive resin compositions because of increased micronization of printed circuit boards in recent years, and from the viewpoint of productivity there is a demand for increased sensitivity of photosensitive resin compositions to allow shorter exposure times.
With the micronization of interconnect pitches of printed circuit boards in recent years, higher resolutions for dry film resists are in demand. Higher sensitivity is also desired from the viewpoint of improving productivity. On the other hand, various types of exposure systems are used depending on the purpose, and maskless exposure that does not require a photomask, such as direct imaging with a laser, is increasing in popularity. Maskless exposure is largely divided according to whether the light source is i-line radiation (365 nm) or h-line radiation (405 nm), and the advantages of each are used to fulfill different purposes. For dry film resists, it is necessary to allow use under the same conditions for “both” types of exposure device; i.e., importance is placed on exhibiting approximately the same sensitivity for “both” types of exposure device and to allow formation of resist patterns with high sensitivity, high resolution and high adhesiveness.
Benzophenone, Michler's ketone and their derivatives, that have been widely used as photopolymerization initiators in photosensitive resin compositions for dry film resists, have absorption ranges localized near a wavelength of 360 nm. Therefore, dry film resists employing such photopolymerization initiators have reduced sensitivity the closer the exposure light source wavelength is to the visible light range, and this makes it difficult to obtain sufficient resolution and adhesiveness with light sources of 400 nm and longer wavelengths.
Thioxanthone and its derivatives, as alternative photopolymerization initiators, can form combinations that exhibit high sensitivity for exposure light sources with wavelengths near 380 nm, by appropriate selection of the sensitizing agent. However, it is usually not possible to obtain sufficient resolution even with resist patterns formed using such combinations, and the sensitivity is still reduced for exposure light sources with wavelengths of 400 nm and longer.
Patent document 2 discloses hexaarylbisimidazole and 1,3-diaryl-pyrazoline or 1-aryl-3-aralkenyl-pyrazoline, as photoreaction initiators with high photosensitivity and satisfactory image reproducibility, and describes examples of forming dry film resists. However, when the present inventors prepared dry film photoresists with photosensitive resin layers comprising 1,5-diphenyl-3-styryl-pyrazoline and 1-phenyl-3-(4-methyl-styryl)-5-(4-methyl-phenyl)-pyrazoline, as compounds specifically mentioned in Patent document 2, the compounds remained undissolved in the photosensitive resin layer and could not be used as dry film resists.
Patent document 3 and Patent document 4 disclose examples of using 1-phenyl-3-(4-tert-butyl-styryl)-5-(p-tert-butyl-phenyl)-pyrazoline. However, although these photoreaction initiators exhibit high sensitivity with exposure using an h-line radiation type exposure apparatus with a wavelength of 405 nm, exposure with i-line radiation type exposure apparatuses has not been able to provide sensitivity comparable to exposure with h-line radiation type exposure apparatuses.
For this reason, it has been desired to provide a photosensitive resin composition that exhibits satisfactory compatibility as a photosensitive resin composition for a dry film resist, that has similar sensitivity for both i-line radiation and h-line radiation type exposure devices, and that exhibits satisfactory sensitivity, resolution and adhesiveness, while a photosensitive resin composition that does not generate aggregates during development has also been desired.    [Patent document 1] Japanese Unexamined Patent Publication HEI No. 07-092673    [Patent document 2] Japanese Unexamined Patent Publication HEI No. 04-223470    [Patent document 3] Japanese Unexamined Patent Publication No. 2005-215142    [Patent document 4] Japanese Unexamined Patent Publication No. 2007-004138